Catalysis of alkyl aromatic isomerization



Patented Feb. 12, 1952 UNITED STATES PATENT OFFICE CATALYSIS OF ALKYLAROMATIC ISOMERIZATION Application July 2,1947, Serial No. 758,522

Claims.

The present invention relates to catalytic isomerization of aromaticcompounds containing a plurality of saturated side chains and moreparticularly to catalyzing the shifting of intact aliphatic groups(preferably without isomerization of the groups per se) along or aroundthe benzene ring of an aromatic hydrocarbon. This type of isomerizationof aliphatic-substituted aromatic compounds involves aliphatic toaromatic carbon to carbon bonds and is hereinafter termed isomerizationat the ring or nucleus to distinguish from mere isomerization ofaliphatic to aliphatic carbon bonds in an aliphatic chain or of aromaticto aromatic carbon bonds in the cycle structure of the ring itself.

The invention is particularly applicable to and finds its greatestpresent utility in the isomerization of dialkyl benzenes, more notablydialkyl benzenes in which at least one alkyl group is methyl, andespecially xylenes. However, it has been found that methyl substituentsare more difficult to isomerize at the aromatic nucleus than are longeralkyl substituents, and it will be apparent in the light of thefollowing disclosure that the invention is applicable to otheraliphatic-substituted aromatics and more desirably to dialkyl benzenescontaining from 1 to 4 carbon atoms in each side chain. For the sake ofsimplicity the invention will be illustrated primarily by reference tothe more difiicult isomerization, i. e., catalyzing the shift of methylgroups at the benzene ring as in the xylenes.

Flexibility and economical utilization of alkyl as blending agents oradditives for certain special motor fuels, and the supply of thissuperior blending agent could be enhanced by isomerization of orthoxylene. In still another situation, relatively pure para xylene isdeveloping into an important chemical for manufacture of variousderivatives, and in those xylene plants which specialize in productionof the para isomer there 2 is the need of a suitable process forconverting ortho xylene or meta xylene or both to the para product. Puremeta xylene (as distinguished from mixtures of meta and para xylenes forgasoline blending) as yet has encountered no large market demand, butflexibility of industrial operations to meet any future requirements forthis isomer makes it desirable to provide a process for isomerization ofortho or para xylenes or both to the meta isomer.

This invention involves catalysis of the foregoing desired isomerizationreactions with phosphoric acid. The catalytic action of phosphoric acidin isomerization of this type can be made highly selective for thedesired shifting of substituents at the ring. Thus, the isomerization ofxylenes by phosphoric acid catalysis may be efiected with a minimum ofside reactions, such as disproportionation and formation of trimethylbenzenes and toluene. The catalytic action of phosphoric acid lowers thetemperature at which isomerization at the ring will occur and therebyreduces or avoids undesirable losses by dealkylation, cracking and otherreactions heretofore encountered in various high temperature thermalpressures.

In practicing this invention it is preferred to feed a mixture ofxylenes containing at least one isomer in a proportion different fromthat represented by isomerization equilibrium or formed by theisomerization reaction. This gives a conversion corresponding to thedifference in composition of feed and reaction product, which can berelatively large as desired. Conversion or isomerization by suchdifferentials has the advantage of avoiding difficult and expensivepurification treatments otherwise necessary to supply a feed stock ofany given pure isomer. On the other hand such differential conversionprocesses necessitates adequate suppression of side reaccan be obtainedby removing a substantial proportion of a desired isomer from thereaction product and again subjecting the remaining isomers toisomerization to replace the isomer which has been withdrawn. Repeatedremoval of desired isomer followed by repeated recycle of residualreaction products to the isomerization zone ultimately affordssubstantially complete conversion of any xylene mixture.

Suitable conditions for catalyzing isomeriza tion of alkyl aromatichydrocarbons by means of a phosphoric acid catalyst in accordance withthe present invention will be apparent from the following. A temperatureof from about 600 F. to about 1100 F. normally will be found operativefor isomerization of xylenes. Somewhat lower temperatures areappropriate where higher alkyl groups are involved. Thus, 400l000 F. isappropriate for diethyl and higher alkyl benzenes. For the isomerizationof xylenes, a temperature of from about 700-900 F. is desirable, anddepending upon catalyst activity a temperature of from about '725-850 F.is presently regarded as optimum. It is to be understood that lesssevere conditions, that is, lower temperatures or lower contact times,or both, are acceptable for isomerization of ethyl and higher molecularweight aliphatic side chains. Such higher alkyl groups are more easilyisomerized at the aromatic nucleus than are methyl groups. Likewise,lower temperatures and lower contact time within the ranges given aremore effective with catalysts of higher activity, and loss in catalystactivity may be compensated in part by elevation of reactiontemperatures or increase in contact times or both.

The phosphoric acid catalyst preferably is orthophosphoric acid, anddesirably is of the type known as a solid phosphoric acid. Such solidphosphoric acid catalysts comprise orthophosphoric acid on a suitableporous carrier such as kieselguhr, activated carbon or the like. Theinclusion of small amounts of metal salts as modifiers, for example,zinc, cadmium, or copper phosphates is not precluded. The broaderaspects of the invention embrace liquid phosphoric acid catalysts ofvarious types, and particularly liquid phosphoric acid films onnon-porous carriers, such as quartz.

Maintenance of acid catalyst strength at approximately 100%orthophosphoric acid by addition of steam or water to hydrocarbon feedis preferred. However, phosphoric acid catalyst of lower strength, forexample, 95% acid or of higher acid strength resulting from partialdehydration, for example, 110% orthophosphoric acid are not precluded.

Contact time for a given conversion in the isomerization reaction is afunction of temperature, catalyst activity, and feed composition.Ordinarily a minimum of about five seconds will be required and amaximum contact time of sixty minutes need not be exceeded. Generallyone to fifteen minutes will suffice. It is to be understood thatpressure and space rates fix contact times at a given temperature andthe foregoing contact times are calculated on the basis of a solidphosphoric catalyst containing 60% free space.

Pressure is not critical; reaction normally will be in vapor phase butliquid phase reactions will occur at higher pressures and lowertemperatures within operative ranges. Pressures of from atmospheric to3,000 pounds per square inch are suitable, but from l1,000 pounds persquare 4 inch is more desirable and 250-500 pounds per square inch ispresently preferred.

Space rates may vary from 0.05 to 5.0 liquid volumes of hydrocarbon feedper volume of catalyst per hour. More desirably, space rate is from 0.1to 2.0 and preferably from 0.25 to 1.0 liquid volumes of hydrocarbonfeed per volume of catalyst per hour.

Diluents are not necessary but may be utilized to suppress sidereactions, such as disproportionation. Hydrogen is an excellent diluent,since it also tends to suppress coke formation and increase catalystactivity. Toluene may be utilized as a diluent to reduce any tendency toform disproportion products by side reactions particularly under moresevere operating conditions.

In accordance with the invention ortho xylene may be produced by feedinga xylene fraction containing less than equilibrium proportions of orthoxylene through a phosphoric acid isomerization zone under isomerizationconditions such as above disclosed. The shift of methyl groups along thebenzene ring is catalyzed by the orthophosphoric acid to form orthoxylene and yield a mixture of xylene isomers enriched in ortho xylenecontent as compared with the feed. The reaction mixture is suitablyfractionated to yield a purified xylene fraction which is passed to anortho xylene recovery unit. Ortho xylene or an ortho xylene richfraction may be separated by distillation. Close distillation as bysuperiractionation will yield an ortho xylene product of 95% or higherpurity as a bottoms fraction. However, for some purposes a 70-90%, ormore desirably an -95% ortho xylene rich product is acceptable.

The ortho xylene poor overhead fraction from the superfractionating orother distillation unit may be recycled to the isomerization zone andthereby eifect conversion of additional meta and para xylenes to theortho xylene isomer.

A para xylene product may be prepared by feeding to the phosphoric acidisomerization system a suitable xylene fraction, such as ortho xylene,meta xylene, a mixture of ortho and meta xylene or a mixture of xylenescontaining a smaller proportion of para xylene than is present in theisomerized reaction product. Isomerization emuent may be purified andtreated for recovery of the para isomer. A suitable method for recoveryof para xylene is selective crystallization. The para xylene product maybe crystallized from the mixed xylene fraction by chilling to atemperature of at least about 50 F. and preferably to about 65 F. Whenthe isomeric xylene fraction is free of other hydrocarbon contaminants.temperatures below -67 F. serve to introduce other isomeric xylenes asimpurities in the crystallized para xylene fraction. The presence ofparaffinic and/or other aromatic hydrocarbon constituents lowers thepermissible crystallization temperature. For additional purification theseparated para xylene crystals may be washed with a suitable solventsuch as isopentane and/or melted and subjected to a secondcrystallization. The para xylene poor mother liquor from thecrystallization may be recycled to the phosphoric isomerization zone toincrease the production of the para isomer.

In similar manner the production of meta xylene involves isomerizationof a suitable xylene fraction followed by separation of a meta xyleneproduct from the resulting isomerized mixture. A suitable method forseparating or recovering meta xylene from the mixture of xylenes in theisomerization eilluent involves selective sulfonation to selectivelyform a meta xylene sulfonic acid phase and a supernatant oil layercontaining predominantly ortho and para xylenes. These phases areseparated and the meta xylene sulfonic acid layer is converted back tometa xylene by hydrolysis. The meta xylene desirably is removed from thehydrolysis reaction zone by distillation' and residual xylene sulfonicacids may be hydrolyzed to recover ortho and para xylenes. Theserecovered xylenes together with the unsulfonated ortho and para xylenefraction are recycled to the isomerization zone and converted to theselected isomer.

In the drawing, one suitable form of isomeriza- .tion unit forpracticing the process of this invention is illustrated.

As shown in the figure of the drawing, xylene feed enters theisomerization system through inlet line H], is raised to reactiontemperature by preheater II and flows therefrom to a fixed bed,phosphoric acid isomerization catalyst chamber l2. To prevent excessivedehydration of the phosphoric acid catalyst Water or steam is introducedinto the xylene feed via line l3. The amount of water so injected iscontrolled to maintain the phosphoric acid catalyst at desiredconcentrations previously disclosed.

An example of suitable operating conditions in an isomerization chamberusing a solid phosphoric acid catalyst comprising orthophosphoric acidof approximately 100% strength supported on kieselguhr is about 700 F.catalyst temperature and about 500 p. s. i. g. pressure. 0.1 liquidvolumes of hydrocarbon feed per volume of catalyst per hour yields amixture of xylenes containing about 19% para xylene.

The isomerized xylene mixture then flows from catalyst chamber I2through outlet conduit M by way of condenser l6 to awater separator I!where the condensate is allowed to stratify into an upper xylene layerand a lower water layer. The water layer is withdrawn through conduit l8and may be returned to xylene feed through line l9 or discarded, asindicated, by valve controlled line 2|. Xylenes from the water separatorll pass through conduit 22 and heat exchanger 23 to purificationsection.

In the process here illustrated, the reaction mixture is purified firstby fractionation in column 24 to eliminate lower boiling hydrocarbonssuch as toluene and/or any volatile diluent. All or part of the toppedxylene mixture is preferably then fractionated in column 26 to yield axylene heart out as overhead and higher boiling impurities or reactionproducts as bottoms. Thus the reaction mixture enters fractionatingcolumn 24 by Way of inlet line 21 and a toluene overhead cut flows fromthe top of the fractionating column through line 28, condenser 29 toreflux drum 3|, from which controlled portions may be returned tofractionator 24 through valve controlled reflux line 32. .Theremainingtoluene distillate flows from refluxdrum 3! and may be recycled toisomerization feed by way of valve controlled line 33 in order tofurnish a diluent which serves to suppres possible side reactions, suchas disproportionation. A portion of the toluene overhead may bewithdrawn through line 34 as a means of controlling the relativeproportions of toluene diluent and xylene feed.

Heat is supplied to fractionator 24 by any suitable means, here shown assteam coil 36. The topped xylenes flow from the bottom of fractionator24 through outlet line 37 and may pass in A space rate of' controlledline 52.

either or both of two directions. In single pass operation when theamount of higher boiling side reaction products is small, all of thexylenes may flow through valve control line 38, heat exchanger 23,cooler 39 and line 4| to product storage. When the amount of higherboiling impurities or side reaction products are relatively large orwhen a cyclic operation is involved, a xylene heart out will be taken byconducting the xylene bottoms from fractionator 24 through valvecontrolled line 32 to fractionator 26. Impurities tend to accumulate orbuild up in a cyclic system such as previously described, i. e., one inwhich the xylenes from the isomerization are passed through an isomerseparation stage and residual xylenes recycled to isomerization. In sucha mode of operation it will be found desirable to control buildup ofheavier or higher boiling impurities, even though original impuritycontent is small for single pass operation, by passing at least aportion of the topped xylene fraction from 'fractionator 24 to andthrough fractionator 26. I

Topped xylenes entering fractionator 26 from inlet line 42 are distilledoverhead and flow through outlet conduit 43, condenser 44 to reflux drum36. A portion of the overhead may be returned to the top of column 26 byvalve controlled reflux unit 4'! and the remainder passes by way of line48 to product storage or isomer recovery treatment, as desired.

Bottoms from fractionator 26 are discharged by Way of line Q9 and may beremoved from the system through valve controlled outlet 5|. In variousinstances it may be found desirable to recycle controlled amounts ofthese heavier impurities to the isomerization feed by way of valve Thismode of operation is particularly useful when and if the isomerizationreaction is conducted under relatively severe con-' ditions which tendto increase the amount of disproportionation products formed. Thistendency to form disproportionation products may be suppressed orreduced by recycle of such disproportionation products from the bottomof fractionating column 26.

Reference has been made to the provision of diluents in theisomerization reaction zone. The presence of diluents miscible with thehydrocarbon reaction mixture increases the selectivity for, orselectively promotes the isomerization reac-- tion, i. e., the samefractional conversion of meta or ortho xylene to para xylene, forexample, are obtained in diluted solutions, as in more concentrated onesunder otherwise comparable conditions. By contrast, second orderreactions such as disproportionation, are selectively reduced orinhibited by such diluents. Toluene as a diluent reduces losses bydisproportionation of xylenes over and above the reduction obtained withsubstantially inert diluents, such as propane or cyclopentane. For thesereasons it is preferred to effect the isomerization reaction in thepresence of a diluent, more desirably toluene, with xylene isomerizationwhen and if the isomerization conditions are sufii'ciently severe tootherwise cause disproportionation of objectionable magnitude. Withdialkyl benzenes other than xylenes, the preferred diluent is amonoalkyl benzene in which the alkyl group is like an alkyl group of thedialkyl benzene being isomerized.

Suitable proportions of diluent usually are in excess of 10% anddesirably in the order of 50% by volume based on the total hydrocarbonfeed. Preferably toluene will be present in proportions notsubstantially less than those formed by dis- :proportionationtoequilibrium, e. g., to 60% by :volume of xylenes.

:Further 'to guide those skilledin the art in the practice of thisinvention and to exemplify the catalytic action of phosphoric acid inthe isomerization .of dialkyl benzenes, illustrative data :are given inTable I.

TABLE I Xylene isomerization Example No (1) (2) (3) OperatingConditions:

, Tempe-m ture, F

Pressureyp. i. g. Feed Raioliquid V/V/Hr Dilueut/Xylene, Mole Ratio.

p-x Composition of Xylene Fraction:

Ethyl Benzene 4 o-xylcne 76.

m-xylene. p-xylene In all of the examples of Table I the catalyst was ofthe solid phosphoric acid type, more particularly ortho phosphoric acidon a kieselguhr support.

The applicability of phosphoric acid catalyst to isomerization of otheralkyl aromatic compounds is illustrated by the isomerization of pcymenewith phosphoric acid on kieselguhr under the following conditions:

Temperature, 510 F.

Pressure, p. s. i. g., 500

Feed rate, liquid V./V./Hr., 1.4

Diluent, toluene Diluent/ xylene, moleratio, 2: 1

Composition of isomerized cyrnene fraction:

ocymene, 3.3% by volume m-cymene, 12.0% by volume p-cymene, 84.7% byvolume.

method for controlling or reducing ethyl benzene build-up may be used.One such method is crystallization oi the xylenes and separation fromethyl benzene which has a lower freezing point. Another method is bydisproportionation of the ethyl benzene and distillation to remove theresulting lower and higher di'sproportionation products. Variouscommercial xylenes also may contain unsulionatable hydrocarbonimpurities commonly designated paraffinic. Such impurities can beeliminated if desired by any suitable treatment of the feed prior toisomerization. In a cyclic type operation either the feed or recyclestock from the xylene isomer recovery stage, or both th fresh xylenefeed and a portion of the recycle stock may be treated for eliminationor reduction of unsulfonatable hydrocarbon impurities. One suitablemethod comprises an extractive distillation process such as disclosed inCope et al. Patent No. 2,215,915, issued September 24, 1940.

' *It is readily apparent from the foregoing description that variousmodifications of the process can be made within the spirit of thepresent invention and the scope of the appended claims. For the sake ofsimplicity and clarity, the drawings have illustrated only major unitoperations in the process and details such as pumps, valves, pressuringmeans, coolers, heat exchangers and the like have been omitted. Anysuitable form of apparatus incorporating these necessary details can besupplied-in obvious manner by thoseskilled in-the'art.

I claim:

1. A process for isomerizing a dialkyl benzene having not more than3carbon atoms in each alkyl group which comprises contacting saiddialkyl benzene with a solid isomerization catalyst containingphosphoric acid at a concentration in the range to 110% calculated asortho-phosphoric acid as the effective catalytic material at atemperature in the range 400 F. to 1100 F. and at a space velocity inthe range 0.05 to 5.0 volumes of liquid dialkyl benzene per volume ofcatalyst per hour the temperature within said temperature range being atleast 600 F. when the dialkyl benzene is Xylene.

2. The method as defined in claim 1, wherein the dialkyl benzene is amethylalkyl benzene having 1 to 3 carbon atoms in the alkyl group.

3. The method as defined in claim 1, wherein the dialkyl benzene is axylene.

4. The method as defined in claim 1, wherein the dialkyl benzene iscontacted with the catalyst in the presence of toluene in amountsufficient to constitute 10 to 50% of the total hydrocarbon charge.

5. A continuous process for isomerizing a xylene feed with a solidisomerization catalyst containing phosphoric acid at a concentration inthe range 95 to 110% calculated as ortho-phosphoric acid as theeffective catalytic material in an isomerization zone at a temperaturein the range to 900 F., at a space velocity in the range 0.1 to 2.0volumes of liquid xylene feed per volume of catalyst per hour, andintroducing into the isomerization zone together with the feed a smallamount of water sufilcient to maintain the concentration of the acidcatalyst within said concentration range.

6. The method as defined in claim 5, wherein the phosphoric acidcatalyst is disposed on a porous solid support.

7. The method as defined in claim 5, wherein the phosphoric acid issupported on kieselguhr.

8. The method as defined in claim 5, wherein the phosphoric acid isdisposed on a non-porous support.

9. The method as defined in claim 5, wherein the phosphoric acid isdisposed on quartz.

10. The method as defined in claim 5, wherein toluene is introduced intothe isomerization zone together with the feed in amount sufiicient toconstitute 10 to 50% of the total hydrocarbon material introduced intothe isomerization zone.

GORDON 'E. LAN GLOIS.

Name Date 2,275,182 Ipatieff et al. Mar. 3, 1942 2,303,547 Hancock Dec.1, 1942 (Other references on following page) Number Number 2,585,899 910 UNITED STATES PATENTS OTHER REFERENCES Name Date Norris et aL, TheRearrangement of Xylenes Carmody et 19, 1946 by Aluminum Chloride, Jour.Amer. Chem. Soc., Reeves July 9, 1946 5 vol. 61 (1939), pages 2131-2-34:(4 pages). Benedict et a1. Apr. 8, 1947 Dobryanskii et aL, MechanismIsomeriza- Passmo et 1947 tion, Oil and Gas Journal, August 8, 1940,page Ipatieff et a1 Mar. 23, 1948 43 (1 page) Howell June 15, 1948Corson Aug. 16, 1949

1. A PROCESS FOR ISOMERIZING A DIALKYL BENZENE HAVING NOT MORE THAN 3CARBON ATOMS IN EACH ALKYL GROUP WHICH COMPRISES CONTACTING SAID DIALKYLBENZENE WITH A SOLID ISOMERIZATION CATALYST CONTAINING PHOSPHORIC ACIDAT A CONCENTRATION IN THE RANGE 95 TO 110% CALCULATED ASORTHO-PHOSPHORIC ACID AS THE EFFECTIVE CATALYTIC MATERIAL AT ATEMPERATURE IN THE RANGE 400* F. TO 1100* F. AND AT A SPACE VELOCITY INTHE RANGE 0.05 TO 5.0 VOLUMES OF LIQUID DIALKYL BENZENE PER VOLUME OFCATALYST PER HOUR THE TEMPERATURE WITHIN SAID TEMPERA-